1. Field of the Invention
The present invention relates to bicycle suspension systems and more particularly to a suspension fork assembly. This invention is improvements on current suspension systems, especially those incorporating the system first described by Farris et al. U.S. Pat. No. 5,320,374. A key objective of this invention is to achieve a longer length of travel and also to allow adjustable travel length of the bicycle suspension system using light weight components while maintaining a reasonable attitude of the bicycle.
2. Description of the Prior Art
Various suspension systems have been proposed and developed for bicycles. Many of these systems utilize a pair of telescoping assemblies between which the front wheel is mounted. Each assembly comprises an outer tube and an inner tube which is free to move in and out of the outer tube and is cushioned by a damper of one sort or another. The outer tubes are connected at the lower ends to the bicycle axle of the front wheel and the upper ends of the inner tube are connected together in a fashion similar to the usual upper end of a bicycle fork.
As is known to those skilled in the art, these types of suspension systems use pairs of anti-friction bushings to allow free movement of the inner tube within the outer tube. These bushings, by themselves, have undesirable static friction called “stiction.” Because of this, the suspension systems using such bushings tend to stick and release. In addition, the two telescoping assemblies also have to be fixed together in some manner as through a “U” shaped yoke at the upper ends of the tubes to eliminate twisting. Even with this “U” shaped yoke the torsional stiffness of these types of assemblies is still limited. These forks are also heavy. They incorporate two sets of tubes, a yoke or other means to eliminate twisting and a steering tube designed to connect to the head tube of the bicycle frame.
A prior art example of a system which overcomes stiction is shown in Farris et al. U.S. Pat. No. 5,320,374 and subsequent applications. In this example an improved form of suspension system is described using an outer tube which is adapted to be mounted in and extend through the head tube of the bicycle frame and an inner tube connected to the fork of the bicycle which telescopes within the outer tube. The inner surface of the outer tube and the outer surface of the inner tube each have a plurality of axially arranged opposing longitudinal flat sections such as four on each tube. A plurality of hardened steel inner race shims are positioned longitudinally on the flats of the inner tube. A plurality of hardened steel outer race shims are positioned longitudinally on the flats of the outer tube. A plurality of needle bearings are disposed between the tubes in between the respective inner and outer race shims. This arrangement allows the two tubes to freely telescope in and out with respect to one another without any significant static friction and also serves to transmit the torsional steering force from the outer tube to the inner tube. This particular system is used extensively today because it can bear a combination of loads comprising very high radial loads and at the same time provide stable and tight rotational motion in steering of the front wheel through the suspension system from the handlebars.
Bicycle riders using suspension systems continue to desire long travel suspension systems to increase plushness, They also desire lightweight systems. Many suspension forks now employ a minimum of 80 mm and the industry trend is to go to 100 mm of travel and greater. In addition, riders would like some form of adjustability to the stroke length of the front suspension systems. Riders would like to shorten the suspension system while traversing uphill so as to lower the attitude of the bicycle while climbing. Subsequently, they would like to lengthen travel once again going on a straight path or downhill to take advantage of the plushness of a longer suspension system. Riders continue to desire to adjust these features at the handlebars vs. leaning over or stopping to make suspension system adjustments.
With the current needle bearing system, several problems exist to incorporate the torsional rigid features it provides with longer travel and suspension length adjustment. Described in the prior art, the highly stressed inner tube must be formed of a material and in a manner such that it bends rather than breaks. High strength steel is commonly used for the inner tube for this purpose, but it is heavy and counter to the consumer's preference. Lighter materials such as Aluminum in combination with strengthening processes such as shot-peening to strengthen the outer skin have been used as a material for the inner tube. Here-to-for, unfortunately, telescope assemblies whose inner tube connects to the fork crown with greater than 70 mm length of travel have been unable to pass stress testing using the present art as described in the aforementioned patents despite the additional costly process of shot-peening and use of expensive high-strength aluminum alloys. Attempting to increase the diameter of the tubes to add strength is also impractical as larger tube diameters increase cost, obsolete existing factory tooling and is generally counter to the consumer's aesthetic preference. Improvements in the design as outlined in U.S. Pat. No. 6,604,886 Kinzler et al have allowed travel to extend to 80 mm using lightweight materials.
Single tube suspension systems mounted in the head tube of the bicycle are unique and preferable over twin tube systems as they eliminate a considerable amount of weight. The main drawback of these systems is their limitation on the length of travel of the suspension system. The longest single telescoping suspension fork system of this configuration presently marketed is 80 mm of travel. Trying to increase this length to 100 mm poses significant problems. The bearing system described by Ferris et al has linear bearings in excess of 4 inches. The length of a 100 mm telescoping suspension portion of the fork using this approach would be in excess of 9 inches putting the attitude of the bike at a steep undesirable angle. Even then the stress on the inner tube member would be so great as to fail during use. There is a limit then on the conventional designs to limit travel to 80 mm or less when connected to a fork style unit. Attempts have been made to use a single-sided suspension system to position the suspension system to one side of the wheel allowing greater travel. While these systems work they are costly and heavy. They also bias the center of gravity of the bicycle pulling the bicycle to one side.
Prior art as described in Farris et. al use a cartridge damper system that exacerbates the length of the suspension portion. Such a restriction adds to the overall length of the telescoping system as the size of the damper components are greater than the space available in the inner tube. The damper commonly extends beyond the inner tube adding to the length of the telescoping system. In addition, such cartridge dampers utilize a coil or air rebound spring located in the inner tube member which places even further restrictions on the stroke length to 80 mm or less.
In the prior art, flats on the outer tube and inner tube of the suspension housing have flats running the entire length of the assembly. This design allowed for hard steel races to be easily installed and for easy installation of the needle bearings from one end. Unfortunately, the race stock is heavy steel and as the suspension system length grows, the length of the race stock grows increasing the weight of the system.
In current designs using the needle bearing system, the radial bearing capacity of the suspension system stops where the linear bearing sits. For much of the travel the bearing is significantly inside the suspension housing allowing flexing of the inner tube. Currently, a collar on the telescope assembly is used to prevent the bearing needles from exiting the telescope at the bottom of its excursion. If the bearing needles escape, the entire front fork may come apart. This is prevented by closing down the internal diameter of the collar. Because of the flexing, however, it cannot be closed down enough to encounter the full length of the bearing cage because the bending of the inner tube when under load may cause it to rub against the collar. A compromise must be made that places severe restrictions on the design, including the outside diameter of the inner tube. As the length of travel is increased this compromise becomes more difficult to make.
Currently no adjustable mechanism exists for Ferris et al. designed suspension forks with the ability to change suspension length at the handlebars.